The objective of this investigation is the radiobiological characterization of tumor cell populations separated and isolated from solid tumors and metastasis for the purpose of eventually integrating this information to develop a predictive assay for tumor response. This study will focus on both intra as well as inter variations in tumor response. Following exposure either in vitro or in situ to radiation alone or in combination with other modalities, selected tumor cell subpopulations will be removed and isolated by biophysical methods. Their responses will be determined and compared to the response of the tumor as a whole. The tumor systems that will be studied include: a methylcholanthrene-induced fibrosarcoma (FSa) and its in vitro growing clone, FSA 1233, a spontaneously arisen fibrosarcoma capable of forming spontaneous metastasis (NFSa), and a spontaneously arisen mammary carcinoma (MCa-K). The cell separation methods used include the separation of cells on the basis of their buoyant densities by means of centrifugation on linear density gradients of either Renografin or Percoll, and on the basis of their size by means of centrifugal elutriation. Each of the separated tumor populations will be characterized with respect to selected biological parameters using biochemical assays, isotope labeling procedures, and flow cytometry in order to better understand its previous environmental situation within the tumor. In particular, individual separated cell populations will be studied following either in situ or in vitro exposure to determine: a) response to radiation, b) response to radiation sensitizing electron-affinic drugs, and c) age response to radiation alone or in combination with other therapeutic agents. To apply these findings to the development of a predictive assay for tumor response, a needle biopsy method will be developed. Density distributions of cells from biopsies will be compared to distributions from their corresponding source tumors to demonstrate the applicability of this technique for evaluating the relative composition of sensitive and resistant, hypoxic or quiescent cells in in situ tumors. These results will be interpreted in relation to designing optimum therapeutic protocols.